Use IBM Plex Sans / Lilex (#36084)

The Zed Plex fonts were found to violate the OFL by using the word Plex
in the name.

Lilex has better ligatures and box-drawing characters than Zed Plex
Mono, but Zed Plex Sans should be identical
to IBM Plex Sans.

Closes #15542
Closes zed-industries/zed-fonts#31

Release Notes:

- The "Zed Plex Sans" and "Zed Plex Mono" fonts have been replaced with
"IBM Plex Sans" and "Lilex". The old names still work for backward
compatibility. Other than fixing line-drawing characters, and improving
the ligatures, there should be little visual change as the fonts are all
of the same family.
- Introduced ".ZedSans" and ".ZedMono" as aliases to allow us to easily
change the default fonts in the future. These currently default to "IBM
Plex Sans" and "Lilex" respectively.

crates/markdown/examples/markdown.rs

@@ -77,16 +77,16 @@ impl Render for MarkdownExample {
     fn render(&mut self, _window: &mut Window, cx: &mut Context<Self>) -> impl IntoElement {
         let markdown_style = MarkdownStyle {
             base_text_style: gpui::TextStyle {
-                font_family: "Zed Plex Sans".into(),
+                font_family: ".ZedSans".into(),
                 color: cx.theme().colors().terminal_ansi_black,
                 ..Default::default()
             },
             code_block: StyleRefinement::default()
-                .font_family("Zed Plex Mono")
+                .font_family(".ZedMono")
                 .m(rems(1.))
                 .bg(rgb(0xAAAAAAA)),
             inline_code: gpui::TextStyleRefinement {
-                font_family: Some("Zed Mono".into()),
+                font_family: Some(".ZedMono".into()),
                 color: Some(cx.theme().colors().editor_foreground),
                 background_color: Some(cx.theme().colors().editor_background),
                 ..Default::default()